1. Field of the Invention
This invention relates to a cornea shape measuring apparatus for ophthalmology in which index marks are projected onto the cornea of an eye to be examined and the reflected image by the cornea is measured to thereby measure the shape of the cornea.
2. Related Background Art
Heretofore, it has usually been the case with a cornea shape measuring apparatus of this type that a ring-like index mark image, i.e., a Mire image, is projected onto the cornea of an eye to be examined and the size or distortion of a reflected image formed by corneal reflection is measured and the radius of curvature of the cornea is calculated.
However, in such a prior-art system, if the spacing between the ring-like index mark and the cornea of the eye to be examined, i.e., the working distance, differs due to the setting error or the like of the apparatus during measurement, the size of the corneal reflection image varies and therefore, a measurement error occurs.
As a method of solving the problem that the size of the corneal reflection image varies depending on the adjustment of such working distance to cause the occurrence of a measurement error, it has been proposed to project the ring-like index mark substantially from infinity through a ring-like cylindrical lens as described in U.S. Application Ser. No. 416,355, U.S. Application Ser. No. 520,217 and U.S. Application Ser. No. 543,041 all now abandoned.
Generally, however, the use of a ring-like cylindrical lens leads to an increased cost of manufacture.
Further, where various concentric index marks of different radii as described in U.S. Ser. No. 520,217 are projected to measure the shapes of the central and marginal portions of the cornea of the eye to be examined, it becomes impossible to make the apparatus compact in a direction perpendicular to the optic axis.